1. Field of the Invention
The present invention generally relates to a superconducting AC current limiter for limiting an overcurrent flowing through an AC (alternating current) power transmission line. More specifically, the present invention is directed to a superconducting AC current limiter equipped with trigger coils made of a superconductor material, which can be quickly recovered to the superconductive state.
2. Description of the Related Art
As is known in an AC power transmission line, e.g., 3-phase power transmission line, when a shortcircuit or a ground fault happens to occur, a failure current will be increased up to several tens to several hundreds killoamperes, which will cause serious damages to various electric appliances connected to the transmission line. To avoid such power failure damages, various current limiters have been proposed. For instance, superconducting current limiters are very recently utilized so as to immediately detect and suppress such very high failure currents.
Various sorts of superconducting current limiters have been proposed in, for instance, a copending U.S. patent application Ser. No. 379,117 filed on Jul. 13, 1989 entitled "SUPERCONDUCTING SWITCH AND CURRENT LIMITER USING SUCH A SWITCH" assigned to one of the present assignees, now U.S. Pat. No. 5,021,914 and U.S. Pat. No. 4,700,257 to Bekhaled, entitled "SUPERCONDUCTIVE AC CURRENT LIMITER". Also, another different type of superconducting current limiter has been disclosed in Japanese patent application No. 1-283200, filed on Oct. 31, 1990 assigned to one of the present assignees.
Furthermore, there is shown in FIG. 1, another conventional superconducting current limiter 3 using a superconductive conductor. The superconducting current limiter 3 is interposed between an AC power supply 1 and a load 4 via a circuit breaker 2. This superconducting current limiter 3 is mainly constructed of a superconductive current limiting coil 3a, and two superconductive trigger coils 3b and 3c which are connected in parallel with each other, and also in parallel with the current limiting coil 3a via a trigger switch 3d. This trigger switch 3d is turned ON/OFF by a quenching sensor 3g. Symbols T.sub.1, T.sub.2, T.sub.3 indicate terminals of this conventional current limiter 3.
FIG. 2 illustrates an internal construction of the superconducting current limiter 3 arranged by mainly the above-described current limiting coil 3a, and trigger coils 3b and 3c. As apparent from this drawing, the conventional superconducting current limiter 3 is so constructed that the current limiting coil 3a and trigger coils 3b, 3c are coaxially wound on a core 51; two trigger coils 3b and 3c are wound inside the current limiting coil 3a under non-electromagnetic induction condition (so-called "Ayrton-Perry windings") by which electromagnetic forces are mutually produced in opposite directions; and both ends of these trigger coils 3b and 3c are connected to each other to constitute a single parallel coil circuit. One terminal of this single parallel coil circuit is connected to one end of the current limiting coil 3a. Then, the current limiting coil 3a will constitute such a superconducting reactor having a predetermined inductance, whereas the trigger coils 3b and 3c will have a superconducting switch function being quenched by a predetermined AC current value. The above-described non-electromagnetic induction state may be understood from FIG. 3. That is, the magnetic fluxes .phi..sub.a, .phi..sub.b, .phi..sub.c produced from the current limiting coil 3a, and trigger coils 3b and 3c, respectively, mutually intersect with each other, and since the trigger coils 3b and 3c are wound in a very close relationship with each other and in the opposite winding direction, these magnetic fluxes .phi..sub.b and .phi..sub.c induced from the trigger coils 3b and 3c are canceled with each other under the normal current, resulting the non-electromagnetic induction state.
In the conventional superconducting current limiter with the above-described arrangement, during the normal conduction (i.e., no overcurrent, or failure current), the overall AC current supplied from the AC power supply 1 passes through the breaker 2, and thereafter most of this current substantially equally flows through these superconducting trigger coils 3b and 3c, but substantially no current flows through the current limiting coil 3a. As a result, a voltage across the current limiting coil 3a becomes very small, so that the quenching sensor 3g for monitoring this voltage is not brought into the active condition. Under such a condition, assuming now that either a shortcircuit failure, or a ground line failure happens to occur in the load 4 and thus a very large failure current may substantially flow through this load 4, the trigger coils 3b and 3c are quenched when this failure current reaches the critical current value of the trigger coils and then represent higher resistance values. Once the trigger coils 3b and 3c are quenched and represent the higher resistance values, most of the circuit current (i.e., failure current) which have flown through the trigger coils 3b and 3c, immediately change their paths. That is, most of the failure current flows through the current limiting coil 3a. As a consequence, this failure current may be reduced to an allowable current value due to the reactor effect caused by this current limiting coil 3a. Then, a voltage appears on both ends of this current limiting coil 3a, which is directly proportional to a product made by this reduced current value and the impedance of the current limiting coil 3a. This voltage is detected by the quenching sensor 3g. As a result, the quenching sensor 3g enables the trigger switch 3d to be opened, so that a small current flowing through the trigger coils 3b and 3c may be interrupted to reduce Joule heat generated by the trigger coils to zero. Accordingly, this may cause quick recovery of the coils to the superconductive state.
In the above-described conventional superconducting current limiter, when the current limiting coil is brought into the current limiting state, most of the magnetic flux .phi..sub.a produced from this current limiting coil 3a intersect the trigger coils 3b and 3c, as readily apparent from FIG. 2. As a result, when the trigger switch 3d is opened by the quenching sensor 3g, the electromagnetic force "e.sub.TC " is simultaneously induced in the trigger coils 3b and 3c in the direction along which the magnetic fluxes .phi..sub.a may be canceled by this force, and then the loop current "i.sub.TCL " flows through the trigger coils 3b and 3c, as represented in FIG. 1. The value of this loop current "i.sub.TCL " is defined by a ratio of the electromagnetic force "e.sub.TC " to an internal impedance "Z.sub.TCt " of the trigger coil and given by the following equation (1): ##EQU1##
Then, the internal impedance "Z.sub.TCt " of the trigger coil is defined by a vector summation among self-inductances L.sub.1 and L.sub.2 of the trigger coils 3b and 3c, a mutual inductance M and restance values of quenched single superconducting coil lines R.sub.1 and R.sub.2, and is given by the following equation (2): ##EQU2## where, .omega.=2.pi.f.
The more, a degree of the superconducting recovery effect of the trigger coils 3b and 3c progresses, the smaller this internal impedance Z.sub.TCt becomes. In other words, the larger, the superconducting area of the single coil line becomes along its longitudinal direction, the smaller the internal impedance becomes. As a result, the loop current "i.sub.TCL " flowing through the trigger coils 3b, 3c conversely becomes high, and thus the Joule heat loss of the coil line region under non-superconducting state is increased. This may cause various problems. That is, cooling helium used to refrigerate this current limiter 3 which is entirely surrounded by this liquid helium, is highly consumed, and furthermore, the recovery time of the trigger coils to the superconducting state is delayed. In addition, the electromagnetic force "e.sub.TC " induced by this loop current may cancel the electromagnetic flux .phi..sub.a induced by the current limiting coil 3a, whereby the resultant indictance of the current limiting coil 3a is lowered.